High speed data transmission is required in the field of middle- to long-range communication (specifically, FTTH (Fiber To The Home) communication, in-vehicle communication, and the like). In order to realize this, optical fiber cables have been used as transmission media.
High speed data transmission is also required in short-range communication (for example, communication within 1 meter). In such a short-range communication field, performances that are difficult to realize with optical fiber cables are also required. Specifically, examples of the requirements include high-density wiring such as a narrow pitch, branching, crossing, or multi-layer, surface mounting properties, integration with an electrical circuit substrate, and bending with a small radius of curvature. A wiring substrate including an optical circuit can be used as one that satisfies these requirements.
In addition, in the optical circuit, an inclined surface capable of reflecting light is formed in a core portion or the like of the optical circuit in order to deflect light at a desired angle to input and output light, for example. Moreover, the wiring substrate including such an optical circuit preferably includes an electrical circuit in order to drive a light-emitting element such as a vertical-cavity surface-emitting laser (VCSEL), a light-receiving element such as a photodiode (PD), and a semiconductor element such as an integrated circuit (IC). That is, the wiring substrate is preferably an optical-electrical composite wiring substrate that includes the optical circuit and the electrical circuit. An example of such an optical-electrical composite wiring substrate includes a bendable optical-electrical composite flexible circuit substrate. The optical-electrical composite flexible circuit substrate is attracting attention and can be used instead of a flexible wiring substrate disposed with a hinge of a small terminal device interposed.
Non Patent Document 1 discloses an example of such an optical-electrical composite flexible circuit substrate.
Non Patent Document 1 discloses an optical-electrical composite flexible circuit substrate in which an optical waveguide film serving as an optical wiring layer and an electrical flexible wiring substrate serving as an electrical wiring layer are laminated. In the optical-electrical composite flexible circuit substrate, the optical waveguide film is laminated on a surface of the electrical flexible wiring substrate, with this surface being on a side where an electrical circuit is not provided, such that an electrical circuit formed in the electrical flexible wiring substrate can be connected to a light-emitting element.
According to Non Patent Document 1, it is possible to obtain an optical-electrical composite flexible circuit substrate having excellent optical properties such as excellent insertion loss and high reliability such as small betiding deterioration.
In addition, the optical-electrical composite flexible circuit substrate requires higher reliability. Specifically, as described above, since the optical-electrical composite flexible circuit substrate is expected to be used instead of the flexible wiring substrate disposed with the hinge of a small terminal device interposed, the optical-electrical composite flexible circuit substrate is required to have smaller bending deterioration (that is, higher bending resistance).    Non Patent Document 1: Toru Nakashiba and two others, Hikari/denki fukugo flekishiburu print haisenban [Optical-electrical flexible printed wiring board], Matsushita Electric Works Technical Report, Matsushita Electric Works Ltd, September 2006, vol. 54, No. 3, p. 38-43